Elastomeric perfluoropolymers (i.e. perfluoroelastomers) exhibit excellent resistance to the effects of heat, weather, oil, solvents and chemicals. Such materials are commercially available and are most commonly copolymers of tetrafluoroethylene (TFE) with a perfluoro(alkyl vinyl ether) such as perfluoro(methyl vinyl ether) (PMVE). Often, these perfluoroelastomers also contain copolymerized units of a cure site monomer to facilitate vulcanization. While these copolymers have many desirable properties, including low compression set and excellent processability, their low temperature flexibility is not adequate for all end use applications. One particularly desirable improvement would be a reduction in glass transition temperature (Tg) with an accompanying extension of service temperature to lower temperatures. Tg is often used as an indicator of low temperature flexibility because polymers having low glass transition temperatures maintain elastomeric properties at low temperatures.
U.S. Pat. No. 4,894,418 discloses vinylidene fluoride based fluoroelastomer compositions which contain processing adjuvants that are perfluorinated polyethers having an hydroxyl group at one or both chain ends. Among the benefits listed for such adjuvants is improved low temperature properties. Being perfluorinated, the polyethers disclosed in the '418 patent are not very compatible with vinylidene fluoride based fluoroelastomers which contain polar hydrogen atoms. Such incompatibility may cause processability problems and lead to the ready extraction of the polyether by solvents.
U.S. Pat. No. 5,268,405 discloses compositions of TFE/PMVE copolymers which also contain a perfluoropolyether (e.g. DuPont's Krytox® fluorinated oil) for reducing the Tg of the composition.
At high temperatures, the perfluoropolyethers employed in all the above compositions tend to be fugitive. As the level of perfluoropolyether in the compositions decreases, the physical properties of the compositions revert to those of compositions containing no perfluoropolyether.
It would be an improvement to have a perfluoroelastomer composition wherein the perfluoropolyether is less fugitive than the above compositions and that also provides the needed reduction in Tg.